INVESTMENT SECURITIES - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	18,892	19,714
After five years but within ten years	186,164	170,118
After ten years	1,662,302	1,034,395
Amortized Cost	1,867,358	1,224,227
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	19,000	19,423
After five years but within ten years	189,078	163,731
After ten years	1,700,743	1,021,925
Available-for-sale securities, fair value total	1,908,821	1,205,079
Held to Maturity, Amortized Cost
Within one year	2,659	1,018
After one year but within five years	5,251	6,703
After five years but within ten years	32,696	29,613
After ten years	94,355	112,616
Debt Securities, Held-to-maturity, Total	134,961	149,950
Held to Maturity, Fair Value
Within one year	2,666	1,016
After one year but within five years	5,280	6,701
After five years but within ten years	33,345	29,547
After ten years	96,971	112,167
Held-to-maturity securities, fair value total	$ 138,262	$ 149,431